Cal. Pub. Util. Code § 180051

(a) A board of supervisors that chooses to create an entirely new entity as an authority pursuant to Section 180050 shall determine the membership of the authority with the concurrence of a majority of the cities having a majority of the population in the incorporated area of the county.
(b) Each member of the authority, and each alternate designated pursuant to subdivision (c), shall be an elected official of a local governmental entity within or partly within the county. Members of the board of supervisors serving on an authority shall comprise less than a majority of the authority.
(c)
1. (1) Each member of the authority may have an alternate to vote or otherwise officially participate on behalf of the member at meetings of the authority when the member is not present. Either the member, or the alternate, but not both, may officially participate in a meeting of the authority. An alternate shall be designated as follows:
  1. (A) Except as specified in subparagraph (B), the local governmental entity that appointed the member shall designate the alternate.
  2. (B) A member who serves because the member holds a specified public office, as specified in the county transportation expenditure plan, shall designate his or her own alternate.
2. (2) An alternate acting on behalf of a member has all of the rights, privileges, and responsibilities of a member.